This invention pertains to an apparatus and method for the nondestructive testing of dielectric materials, particularly to nondestructive testing with microwaves.
There is an unfilled need for improved, nondestructive means to test dielectric materials for flaws, defects, and irregularities.
An example of this unfilled need is that for improved, nondestructive means for inspecting rubber expansion joints. Although this invention has numerous applications and is by no means limited to the inspection of rubber expansion joints, that particular use will be described briefly because it played a significant role in inspiring the conception of this invention.
Most steam-cycle electric power plants employ rubber expansion joints between the condenser and the turbine. The expansion joints have multiple composite layers. Typical dimensions for such an expansion joint are in the neighborhood of 40 meters circumference, by 25 cm wide, by 1 cm thick. Under normal operating conditions, there is a vacuum on the inside of the joint, and 1 atm pressure on the outside. Thus when such a joint fails, it is prone to catastrophic failure.
A defect can begin, for example, when a small crack allows moisture inside the rubber. Moisture can then wick along the cords that form part of the composite. The moisture can cause the cord to deteriorate, which can lead to adjacent layers delaminating from one another. Defects such as these inside a joint are difficult to detect nondestructively through conventional means.
It is highly desirable that a testing procedure be nondestructive, and be usable whether the plant is running or idle. Furthermore, because the access space outside the joint can be as little as 7-10 cm, any portion of the detection machinery that must be in contact with the joint (or in the vicinity of the joint) should be small enough to fit into such a space.
If the joint were made of metal, then well-established ultrasonic inspection techniques could be used. However, ultrasonic inspection cannot be used for rubber or soft plastic, because the polymers absorb nearly all sound energy, and reflect essentially none. The mesh or fabric of a composite material so highly scatters and disperses the ultrasonic waves that an extremely noisy reflection results. Eddy current measurements or magnetic measurements do not work well in rubber either, because rubber does not conduct electricity.
Neither is radiography particularly helpful. X-ray radiation is used to detect changes in bulk density. Under most operating conditions the most common flaw leading to failure is delamination. In a delamination failure, an essentially two-dimensional separation occurs between adjacent component layers. This separation between layers does not typically result in a detectable change in local density, and is therefore not detectable in a radiograph.
The current state of the art for nondestructive testing of rubber parts is to use a Durometer, a needle that penetrates a portion of the rubber, and connects to a strain gauge. Durometers have poor practical utility, but they represent the best technology currently available for non-destructive testing of rubber joints.
An overview of microwave testing techniques may be found in A. Bahr, Microwave Nondestructive Testing Methods (1982).
Several microwave nondestructive testing techniques are disclosed in A. Lucian et al., xe2x80x9cThe Development of Microwave NDT Technology for the Inspection of Nonmetallic Materials and Composites,xe2x80x9d pp. 199-232 in Proceedings of the Sixth Symposium on Nondestructive Evaluation of Aerospace and Weapons Systems Components and Materials (San Antonio, Tex. 1967).
J. Kurian et al., xe2x80x9cMicrowave Non-Destructive Flaw/Defect Detection System for Non-Metallic Media Supported by Microprocessor-Based Instrumentation,xe2x80x9d J. Microwave Power and Electromagnetic Energy, vol. 24, pp. 74-78 (1989) discloses a method for detecting defects in a tire by measuring transmission of microwaves from a dipole transmitting antenna inside the tire, through the treads of the tire, with transmission detected by a linear array of detectors. Differential rates of transmission were correlated with changes in thickness or with defects.
C. Howell et al., The Use of Low Cost Industrial AM-CW xe2x80x98Microwave Distance Sensorsxe2x80x99 for Industrial Control Applications (no date) discloses a microwave distance sensor to measure distances to an object from about 15 centimeters to about 6 meters away, by measuring the phase angle of a returned amplitude-modulated microwave signal reflected from the object.
U.S. Pat. No. 3,278,841 discloses a microwave flaw detection system, particularly for use with large, solid-propellant rocket motors. Microwaves were transmitted from inside the propellant, reflected off a metal casing, and detected by a receiver displaced from the microwave transmitter. Irregularities in the strength of the received signal were correlated with cracks or other flaws in the propellant.
U.S. Pat. No. 4,520,308 discloses a system for measuring the thickness of a dielectric material by measuring the phase shift of microwaves transmitted along a microwave strip line conductor that is adjacent to the material whose thickness is being measured. See also U. S. Pat. No. 4,123,703.
U.S. Pat. No. 2,999,982 discloses a Doppler-effect-based method for microwave detection of homogeneity defects in compact materials such as polished glass. Relatively high speeds of scanning were used to generate the desired Doppler effect. In the one example given, the relative speed of the glass versus the detector was 650 centimeters per second.
U.S. Pat. No. 3,144,601 discloses a method for microwave detection of non-homogeneous zones in non-conducting materials such as glass sheets and plates. Detection was performed by simple measurement of the echoes of the reflected microwaves; by measuring losses in intensity following transmission through the object; or by mixing incident and reflected waves to create beats, particularly when the material being examined was traveling (i.e., detecting Doppler shifts in the frequency of the reflected microwaves).
U.S. Pat. No. 3,271,668 discloses the use of microwaves to measure the rate of progressive attrition from a surface of a body of a solid dielectric material; for example, measuring the burning profile in a solid rocket motor. Microwaves were transmitted through the fuel (or other material), the surface of which reflected some of the microwaves back to a detector. The relative phase of incident and reflected microwaves varied as the distance from the microwave transmitter to the surface of the burning fuel changed, allowing the distance to the surface of the fuel to be determined as a function of time.
U.S. Pat. No. 4,707,652 discloses a technique for detecting impurities in a bulk material by measuring changes in the scattering of microwave radiation incident on the bulk material.
U.S. Pat. No. 4,514,680 discloses a method for detecting knots in lumber, by transmitting microwaves through the lumber from two sources of the same intensity, but with a 180xc2x0 phase shift. Transmitted microwaves are detected on the opposite side of the lumber. If the lumber is knot-free, there is a null in the microwave field at the detectors, but if a knot is present the phase and amplitude of microwave radiation at the detectors are altered.
U.S. Pat. No. 4,581,574 discloses a method for determining the average dielectric constant of a dielectric material having a conductive surface, by transmitting microwaves from two transducers into a sheet of the material, and making measurements of the energies of reflected microwaves. By measuring average dielectric constants along a plurality of paths in the plane of the sheet, locations of variations within the sheet may be identified.
U.S. Pat. No. 4,274,288 discloses an acoustic, interferometric method for measuring the depth of a surface flaw such as a crack.
U.S. Pat. No. 4,087,746 discloses a method for determining optical anisotropy in a dielectric material by measuring changes in the polarization of microwaves transmitted through the material.
A novel apparatus and method have been discovered for the nondestructive inspection of dielectric materials. Monochromatic, phase coherent electromagnetic radiation, preferably in the 5-50 gigahertz frequency range (i.e., microwave radiation) impinges on the sample. In accordance with Snell""s law, the microwaves are partly transmitted and partly reflected at each interface where the dielectric constant changes (e.g., where there are delaminations, cracks, holes, impurities, or other defects.)
A portion of the transmitted beam is combined with the signal reflected by the specimen being inspected. These two signals have the same frequency, but may differ in amplitude and phase. The signals combine to produce an interference pattern, a pattern that changes as the specimen changes, or as the position of the specimen changes relative to that of the detector. Appropriate processing of the interference signal can greatly improve the signal-to-noise ratio. The detector may be scanned relative to the specimen at any desired speed, and the scanning speed need not be uniform. The novel detection technique is not based on Doppler-shifts in frequency, which result from motion, but rather is based on interference between reflected and reference microwaves having substantially the same frequency, where the interference is caused by changes in location (independent of motion per se).
The novel technique can detect cracks, voids, foreign material inclusions (e.g., water or oil), thickness changes, delaminations, changes in dielectric constant (which in rubber may, for example, indicate aging), and other defects in essentially any dielectric materials. Different types of defects have distinguishable characteristics. The technique can also be successfully used on composite materials containing conductive components, but whose construction makes them overall nonconductorsxe2x80x94for example, carbon fiber composites.
Substances such as fiberglass that produce noisy reflection patterns in prior ultrasonic techniques may be inspected at low noise levels with the novel microwave technique. The novel technique readily detects many common defects in fiberglass.